This invention relates to an electronic circuit and more particularly, to a phase comparison circuit and other electronic circuits including a current mirror circuit.
A phase comparison circuit, for example, can utilize an electronic circuit including a current mirror circuit. The circuit can function as a pilot signal detecting circuit in a PLL-MPX (phase locked type-multiplex) decoder IC which demodulates a stereophonic signal. Such a pilot signal detecting circuit is shown in FIG. 1. As shown, a stereophonic signal, which comprises main and sub-audio signals and a pilot signal, is supplied across the base electrodes of transistors Q.sub.1, Q.sub.2. Transistors Q.sub.1, Q.sub.2 constitute a differential amplifier 11. After the stereophonic signal is amplified by amplifier 11, it is supplied from the collector electrodes of Q.sub.1, Q.sub.2 to each common base electrodes of transistors Q.sub.3, Q.sub.4 and transistors Q.sub.5, Q.sub.6. Transistors Q.sub.3 to Q.sub.6 constitute a double balanced differential amplifier 12. A switching signal from a switching signal source 13, which has the same frequency as the pilot signal (e.g., 19KHz) is supplied across the base electrodes of transistors Q.sub.3, Q.sub.6 and transistors Q.sub.4, Q.sub.5. Amplifier 12 multiplies the stereophonic signal by the switching signal and produces an output signal from the collector electrodes of transistors Q.sub.4, Q.sub.5 to an output terminal 15. An AC component in the output signal is supplied to power supply line L.sub.11 through a condenser C.sub.11, connected between the terminal 15 and line L.sub.11. Consequently, a DC level corresponding only to the pilot signal is provided from terminal 15 to a stereo indicative lamp (not shown).
Voltage fluctuation in the power supply are reduced by a current mirror circuit 14, positioned between amplifier 12 and L.sub.11, which comprises a pair of transistors Q.sub.7 and Q.sub.8. Although it is desired for transistors Q.sub.7, Q.sub.8 to have identical characteristics so the current can be balanced, this is generally not possible due to the mass production of transistors. In general, therefore, an error current flows from mirror circuit 14 to output terminal 15. If the current gain of circuit 14 is 1+.epsilon., the error current flowing into terminal 15 is (.epsilon./2)I.sub.o ; where .epsilon. is the error factor due to the unbalance of transistors Q.sub.7, Q.sub.8, and I.sub.o is a current value produced by current source I.sub.11. Source I.sub.11 is connected to the common emitter electrode of transistors Q.sub.1, Q.sub.2. In general, .epsilon. equals (approximately) .+-.0.1; as a result, 5% of the current value I.sub.o is the error output current supplied to terminal 15.
This error output current, however, is not negligible. In the stereophonic signal, the modulation factor of the pilot signal is 10% and that of each audio signal is 90%. Accordingly, the level of each audio signal is nine times the level of the pilot signal. Furthermore, the current value I.sub.o need be large to secure the dynamic range of the stereophonic signal. If I.sub.o is 600 .mu.A, the variation of the pilot signal becomes approximately .+-.10 .mu.A provided that the upper and lower dynamic ranges of the stereophonic signal is three times the level of the audio signal. If I.sub.o is 600 .mu.A, then the error signal current becomes approximately .+-.30 .mu.A. Consequently, the output including a large error component, in comparison with the pilot signal, will be supplied from terminal 15 to the stereo indicative lamp even when a stereophonic program is not be received. Accordingly, this high level error signal should not be detected.